Advanced search with distances using NHibernate an

Question

I've got an existing advanced search method in a repository that checks a FormCollection for the existence of search criteria, and if present, adds a criterion to the search e.g.

public IList<Residence> GetForAdvancedSearch(FormCollection collection)
{
  var criteria = Session.CreateCriteria(typeof(Residence))
    .SetResultTransformer(new DistinctRootEntityResultTransformer());

  if (collection["MinBedrooms"] != null)
  {
    criteria
      .Add(Restrictions.Ge("Bedrooms", int.Parse(collection["MinBedrooms"])));
  }

  // ... many criteria omitted for brevity

  return criteria.List<Residence>();
}

I've also got a basic distance search to find how far each residence is from the search criteria. The HBM for the query is

<sql-query name="Residence.Nearest">
  <return alias="residence" class="Residences.Domain.Residence, Residences"/>
  <return-scalar column="Distance" type="float"/>
  SELECT R.*, dbo.GetDistance(:point, R.Coordinate) AS Distance
  FROM Residence R
  WHERE Distance < 10
  ORDER BY Distance
</sql-query>

I had to define a function to calculate the distance, as there was no way to get NHibernate to escape the colons in the geography function:

 CREATE FUNCTION dbo.GetDistance
 (
  @firstPoint nvarchar(100), 
  @secondPoint GEOMETRY
 )
 RETURNS float
 AS
 BEGIN
  RETURN GEOGRAPHY::STGeomFromText(
  @firstPoint, 4326).STDistance(@secondPoint.STAsText()) / 1609.344
 END

And the repository calls the named query thus:

return Session
   .GetNamedQuery("Residence.Nearest")
   .SetString("point", String.Format("POINT({0} {1})", latitude, longitude))
   .List();

So my question is; how do I combine the two (or start from scratch), so I can filter the advanced search results to include only residences within 10 miles of the search location?

UPDATE I have tried using NHibernate.Spatial with the following code:

criteria.Add(SpatialExpression.IsWithinDistance(
    "Coordinate", new Coordinate(latitude, longitude), 10));

but SpatialExpression.IsWithinDistance returned a System.NotImplementedException.

Answer 1

Create a projection that, in effect, adds a new distance column to the results, which is calculated by called the UDF, and then add a restriction to it:

var query = String.Format(
    "dbo.GetDistance('POINT({0} {1}', Coordinate) AS Distance", 
    latitude, longitude);
criteria
    .Add(Restrictions.Le(Projections.SqlProjection(
        query, 
        new [] {"Distance"}, 
        new [] {NHibernateUtil.Double}), 10));

UPDATE

n.b. Although this must have worked when I posted it, it doesn't work any more. NHibernate doesn't like the '.' after dbo, and says

"could not resolve property: dbo of: Residences.Domain.Residence".

If I remove the 'dbo.' I get

"'GetDistance' is not a recognized built-in function name."

Answer 2

Have you seen the NHibernate.Spatial project? This may provide an easy solution to your problem.

The alternative is to create your own implementation of ICriterion - this is not too tricky if you derive from AbstractCriterion and you target your particular database platform. This would then allow you to combine your distance function with other criteria.